Cosmetic compositions containing an antiperspirant that are intended for topical application to the body are available to the public in a number of product forms, of which one form that has been popular in the last 10 years is that of soft solids. These have sometimes alternatively been referred to as semi-solids or as anhydrous creams. In essence, and irrespective of the name used, the soft solid comprises a particulate antiperspirant active that is dispersed within a water-immiscible carrier fluid that is structured by one or more structurants. It is commonly stored in a dispensing container which comprises a product chamber, usually a round or oval cylinder, having at one end a cover defining a number of apertures through which the contents of the chamber can be dispensed and a means to expel the contents through the apertures. That means can comprise squeezable side-walls, or more commonly a platform or elevator at an intermediate position within the cylindrical chamber that can be advanced towards the first end.
In order for the soft solid to be dispensed through the aperture(s) in the cover of the chamber, the rheological properties of the composition are altered when it is subjected to mild pressure and especially in the vicinity of the aperture wall compared with the normal state that existed prior to the soft solid being extruded under pressure through those apertures. When pressurised and extruded through the aperture, the micro-structure of the solidified carrier liquid is partly destroyed adjacent to its contact boundary with the aperture, so that the viscosity of the composition is locally reduced, thereby enabling the composition to flow. This subsequently becomes especially noticeable in the vicinity of the apertures through which the composition is being dispensed. The micro-structure of the composition that has been extruded through the apertures or that which remains within or close to the apertures may often not reform to a significant extent or at best reform only extremely slowly when the pressure on the composition is released, with the result that after the composition has been subjected to pressure and partial extrusion, it still has a lower viscosity in the vicinity of the cover apertures. Indeed, a separate localised liquid phase can be formed, particularly from one or more of the oils in which particulate materials are suspended. An undesired consequence of the formation of a separate liquid phase is a problem called syneresis, in which the liquid phase, particularly the oils, can seep out of the chamber, or possibly be expelled from the chamber as a consequence of residual or subsequent pressure in the container, onto the surface of the dispensing container, and possibly trickling down the sides of the container.
Although damage to the microstructure of the soft solid occurs at the time of extrusion through the cover apertures, the appearance of the oils or other liquid on the surface tends to be retarded, because it takes a finite period of time for transfer of fluid from within the container. Thus, the extent of liquid on the container surface tends to increase for a period of maybe half an hour to 2 or 3 hours after the product has been dispensed (and applied topically to the armpit) depending on the oils and the structurants employed. Thereafter, there tends to be little, if any, additional egress of oil. However, once the oil has egressed, there is no mechanism for its subsequent recapture by the remaining composition within the container. Moreover, since the problem of syneresis does not occur instantly, but develops during the period of at least half an hour later, the consumer normally does not have an opportunity to take remedial action by wiping the container after the oils have appeared. If wiping were done shortly after use, the problem would materialise subsequently. Consequently, whether on the top or sides, the oils can form a sticky and often unsightly mess, possibly after partial evaporation. This is not liked by consumers who wish to use dispensers that are not sticky to hold or unsightly to view.
Consequently, in one respect, the problem that the instant invention seeks to address is how to formulate a soft solid composition so that the problem of syneresis is eliminated or at least ameliorated.
One strategy that has been proposed by some patentees to combat the problem of syneresis comprises incorporating a pressure-relief mechanism that forms part of or is linked to the mechanism that expels soft solid out of the dispenser comprises means to reduce the extent of pressure generation in the dispenser for soft solids.
These mechanisms can include suck-back mechanisms to automatically partially withdraw the elevator or platform away from the cover immediately subsequent to its advancement, thereby increasing the volume available for the soft solid within the dispenser between the platform and top wall and accordingly reducing the pressure immediately. A second means which mitigates the problem by increasing the ratio of the volume of composition that is extruded relative to the aperture sidewall length comprises increasing the proportion of apertures in the top wall. The two means may be employed together. (U.S. Pat. No. 6,143,284). However, a pressure release mechanism mentioned above can render more complicated the advance mechanism, and increase the cost of the dispenser, which already can constitute a very significant proportion of the total cost of the product. Furthermore, it does not address the fundamental question of how to reduce the inherent susceptibility of the composition to syneresis.
The problem of its syneresis is not the only problem that is potentially associated with a soft solid composition.
In the course of investigating the suitability of a number of antiperspirant compositions to carry out their intended purpose, the instant inventors have recognised that changes to the constitution of the carrier liquid and structurants can not only alter the susceptibility or resistance of the composition to syneresis, but can simultaneously impair the inherent capability of the suspended antiperspirant to control perspiration when the composition has been applied topically onto human skin, such as in axilary regions. Moreover, any resultant composition still needs to be capable of being dispensed through the top wall of the dispenser when subjected to mild pressure. Accordingly, the problem faced by the instant inventors was to devise soft solid compositions that not only enjoyed an improved inherent resistance to syneresis, whilst simultaneously remaining dispensable, but at the same time also maintaining an acceptable capability to control perspiration.